Revisiting aromatic thiols effects on radical photopolymerization (original) (raw)
Related papers
Evaluation of Aromatic Thiols as Photoinitiators
The unique photodynamics of aromatic thiols (relative to alkyl thiols) allowed their employment as effective ultraviolet and visible light photoinitiators (PIs) for acrylate photopolymerizations, radicalmediated thiol−ene coupling (TEC) network polymerizations, solventless and initiatorless small-molecule TEC reactions, and hydrogel network polymerizations. Twenty-six thiol structures were evaluated as radical generating PIs in the polymerization of n-hexyl acrylate. Initiator effectiveness follows the trend nonheterocyclic aromatic thiols > heterocyclic aromatic thiols ≈ thioacids > alkyl thiols. Substituted versions of thiophenol exhibited the highest photoinitiation efficiency with electron-withdrawing substituents increasing effectiveness. Ortho-and para-substituted mercaptobenzoic acids and (trifluoromethyl)thiophenols were generally the most effective, leading to 100% acrylate conversion at a loading of 3 mM (0.1 mol % with respect to acrylate) when irradiated with 320−390 nm light and at loadings of 30 mM when irradiated with 405 nm light. Results demonstrate the potential of aromatic thiols as oxygen insensitive photoinitiators for bulk polymerizations and for specialty polymer synthesis via aromatic thiol-functionalized macroinitiators.
Macromolecules, 2009
A bifunctional photoinitiator for free radical polymerization, thioxanthone catechol-O,O 0diacetic acid, was synthesized, characterized, and compared to photoinitiator parameters of the monofunctional analogue, 2-(carboxymethoxy)thioxanthone. Photophysical studies such as fluorescence, phosphorescence, and laser flash photolysis in addition to photopolymerizations of methyl methacrylate show that the bifunctional photoinitiator is more efficient in polymer generation than the monofunctional derivative. These studies suggest that initiator radicals are generated from a π-π* triplet state in an intramolecular electron transfer, followed by proton transfer and decarboxylation to generate alkyl radicals, which initiate polymerization. The initial electron transfer is faster for the bifunctional photoinitiator than the monofunctional derivative, which is based on laser flash photolysis studies. Because of the relatively fast intramolecular radical generation from the triplet state (triplet lifetime = 490 ns), quenching by molecular oxygen is insignificant and polymerization of methyl methacrylate proceeds efficiently without deoxygenation. At higher concentrations of initiator (∼5 mM) intermolecular electron transfer competes with intramolecular electron transfer. Both processes, inter-and intramolecular processes, yield initiating alkyl radicals.
Macromolecules, 2011
' INTRODUCTION Thioxanthone (TX) and derivatives are among the most widely used type II photoinitiators in various UV curing applications because of their excellent light absorption characteristics. 1À3 In most cases, the photoinitiating free radicals are generated by hydrogen abstraction by the triplet excited state of TX from hydrogen donors such as amines or thiols (Scheme 1). 4À6 However, low-molecular-weight amines, particularly at high concentrations, have several intrinsic disadvantages such as objectionable odor, toxicity, 7 and migration in UV-curing technology and cause a decrease in the pendulum hardness of the cured films. 8 One way to overcome these problems is to chemically incorporate the hydrogen-donating sites into TX chromophores. 9À11 We have previously reported several thiol 12 and acetic acid 13 derivatives of TX as photoinitiators for free radical polymerization. A major advantage of this type of initiator is related to their one-component nature. They can serve as both a triplet photosensitizer and a hydrogen donor.
Macromolecules, 2005
A mechanistic study concerning photoinitiated free radical polymerization using thioxanthone thio-acetic acid (TX-S-CH 2-COOH) as one-component Type II photoinitiator was performed. Steady-state and time-resolved fluorescence and phosphorescence spectroscopy, as well as laser flash photolysis was employed to study the photophysics and photochemistry of TX-S-CH 2-COOH. The initiator undergoes efficient intersystem crossing into the triplet state and the lowest triplet state posseses π-π* configuration. In contrast to the unsubstituted thioxanthone, TX-S-CH 2-COOH shows an unusually short triplet lifetime (65 ns) indicating an intramolecular reaction. From fluoroscence, phosphorescence, and laser flash photolysis studies, in conjunction with photopolymerization experiments, we propose that TX-S-CH 2-COOH triplets undergo intramolecular electron transfer followed by hydrogen abstraction and decarboxylation producing alkyl radicals, which are the active initiator radicals in photoinduced polymerization. At low initiator concentrations (below 5 × 10 -3 M) this intramolecular reaction is the dominant path. At concentrations above 5 × 10 -3 M, however, the respective intermolecular reactions may be operative.
Control of Free-Radical Reactivity in Photopolymerization of Acrylates
2012
In practical use, it is the rate of cross-linked network formation that is of most interest in the free radical curing of acrylates. One of the simplest indicators of the vitrification point is the sharp rise in MEK double rubs (MEKR) that occurs as a function of radiation dose applied. It is well-known that oxygen inhibition and radical-radical recombination limit efficiency, delaying the onset of glassy network formation to higher dose or longer exposure time. This paper explores means to alter the reactivity of the intermediate free radicals, in one case by chain transfer to thiol and in another by complexation with stable nitroxide radicals, thereby altering the population of chain-carrying species to favor longer-lived radicals or radical precursors which by their nature are less affected by the normal termination processes. By chain transfer formation of thiyl radicals, a simple titration of reactive radicals is possible as observed by detection of the point of maximum MEKR as...
Macromolecules, 2009
Radical-mediated thiol-yne step-growth photopolymerizations are utilized to form highly crosslinked polymer networks. This reaction mechanism is shown to be analogous to the thiol-ene photopolymerization; however, each alkyne functional group is capable of consecutive reaction with two thiol functional groups. The thiol-yne reaction involves the sequential propagation of a thiyl radical with either an alkyne or a vinyl functional group followed by chain transfer of the radical to another thiol. The rate of thiyl radical addition to the alkyne was determined to be approximately one-third of that to the vinyl. Chain-growth polymerization of alkyne and vinyl functionalities was only observed for reactions in which the alkyne was originally in excess. Analysis of initial polymerization rates demonstrated a near first-order dependence on thiol concentration, indicating that chain transfer is the rate-determining step. Further analysis revealed that the polymerization rate scaled with the initiation rate to an exponent of 0.65, deviating from classical square root dependence predicted for termination occurring exclusively by bimolecular reactions. A tetrafunctional thiol was photopolymerized with a difunctional alkyne, forming an inherently higher cross-link density than an analogous thiol-ene resin, displaying a higher glass transition temperature (48.9 vs -22.3°C) and rubbery modulus (80 vs 13 MPa). Additionally, the versatile nature of this chemistry facilitates postpolymerization modification of residual reactive groups to produce materials with unique physical and chemical properties.
Initiation and kinetics of thiol-ene photopolymerizations without photoinitiators
Journal of Polymer Science Part A: Polymer Chemistry, 2004
Thiol-ene photopolymerizations without added photoinitiators were studied, and the kinetics accurately predicted by modeling for a wide range of different vinyl chemistries. Initiation rates in polymerizations initiated with light centered around 365 nm were found to be proportional to the concentration of ene functional groups. When 254 nm light was used as the initiating light source, initiation rates were proportional to the concentration of thiol functional groups. The mechanism of initiation at 254 nm was attributed to direct cleavage of thiol functional groups. An appropriate species or mechanism has not yet been found that is consistent with the experimental data for initiation when 365 nm light is used.
Macromolecular Chemistry and Physics, 2013
Ten acridinediones (ADs) with different substituents are prepared and investigated for their abilities to initiate a ring‐opening cationic photopolymerization in combination with an iodonium salt and N‐vinylcarbazole upon UV–vis light (Xe–Hg l& >330 nm) or visible light (halogen lamp) exposure. The structural effects in the ADs are outlined. The cationic and radical photopolymerization of an epoxide/acrylate blend in a one‐step hybrid cure very efficiently leads to the formation of an interpenetrated polymer network under UV–vis light or visible light irradiation (30 s for tack‐free coatings). The photochemical mechanisms are studied by molecular orbital calculations, steady‐state photolysis, electron spin resonance–spin trapping, cyclic voltammetry, fluorescence, and laser flash photolysis techniques. image
New Thiols for Photoinitiator-Free Thiol-Acrylate Polymerization
Macromolecular Chemistry and Physics, 2013
New thiols for effi cient thiol-ene polymerization reactions are presented. They do not exhibit any unpleasant odor, are characterized by quite good light-absorption properties at λ > 300 nm, and generate thiyl radicals upon UV-light exposure. Due to these interesting properties and contrary to many previously reported methodologies, the present thiol-acrylate polymerizations can be effi ciently carried out without the presence of any additional photoinitiator. The chemical mechanisms are investigated by steady-state photolysis and electron spion resonance (ESR) experiments. Important parameters can also be extracted from molecular orbital calculations.
Journal of Polymer Science Part A: Polymer Chemistry, 1997
Relative reactivities of radicals derived from several 1-( N,N-dimethylamino)ethylene-2-derivatives towards the polymerization of butyl acrylate have been calculated. It has been observed that the reactivity of the radicals increases as the electron withdrawing character of the groups attached in the b position to the N atom increases. The experimental efficiency factor ( f ) has been found to be proportional to the s F value defined in the Taft equation. The dependence of the reactivities with the nature of the radicals has also been explained through their frontier orbital characteristics. The presence of heteroatoms next to the radical center could modify, in some cases, the accuracy of these calculations, although the inclusion of more sophisticated quantummechanical methods does solve this drawback. Moreover, the enthalpy calculation of the possible radicals formed from a certain molecule gives a good approach to realize about the existence of anomalous behaviors.